The present invention relates to protective relay circuits for use in AC power distribution systems and more particularly to phase relays for transmission line protection.
Distance relays are well known in the art of protective relaying. For example, reference may be had to "The Art and Science of Protective Relaying", Mason, published by John Wiley and Son, Inc. (1956), particularly to Chapter 14. Prior art distance relays include those which employ a phase comparator method of measurement. Exemplary circuits for phase comparator distance relays are disclosed in U.S. Pat. No. 4,034,269, issued to S. B. Wilkinson, one of the co-inventors of the present invention, on July 5, 1977. This patent is assigned to the assignee of the present invention and is incorporated by reference into the present application as if fully set forth herein.
Phase distance relays are utilized in protective systems for AC power transmission lines to detect phase to phase faults within a protected zone or portion of the transmission line. Phase distance relays are utilized to detect faults involving more than one electrical phase and if such a fault is detected within the relay's zone or reach, the phase relay will cause the operation of a three pole circuit breaker to trip all three phases of the transmission line. This is to be contrasted with a ground relay which detects faults between a phase and ground, again within a designated zone or reach; and, upon detection of such a fault, generates a signal which may operate a circuit breaker to trip the faulted phase only.
As indicated above, each particular phase relay should only detect faults within its protected zone or reach. The parameters of a phase relay are commonly selected to correspond to the parameters of the transmission line. For example, the parameters are selected to provide a forward reach that may correspond to the forward distance of the zone of the transmission line under the protection of the particular relay. It is desired that the phase relay operate within its selected forward reach thereby confining the protection of a particular phase relay to a selected zone within a protected system. A reverse offset may also be used if required.
The desired operation of a phase relay may be hindered by the inability to distinguish between internal and external faults. Internal faults are those faults which occur within the protected zone or reach of the relay, whereas external faults are those that occur outside of the zone or relay reach. For example, transients generated by capacitive voltage transformers or series capacitors at the occurrence of an external fault, which are coupled to the transmission line, often appear as internal faults to a phase relay. Another problem affecting the operation of phase relays is the generation of high frequency transients by travelling waves at the incidence of a fault, or transients occurring when the protective gaps of series capacitors flash over. These high frequency transients may cause undersireable delays in operation for faults within the protected zone of the relay. Also, these high frequency tansients might cause the erroneous generation of a trip signal by a phase relay which improperly recognizes the transient as a transmission line fault within the protected zone of the relay.
An additional problem which can adversly affect the operation of phase relays is the generation of a three phase tripping signal upon detection of a close-in single phase to ground fault. Upon the occurrence of such a single phase to ground fault, it is often desirable only to trip the faulted phase leaving the two remaining unfaulted phases in service. Due to the potential catastrophic consequences of severe internal faults in a high voltage AC power transmission system, it is desirous that a phase relay provide a tripping signal as quickly as possible upon detection of such a severe fault. In most prior art phase relays, the detection of such severe close-in faults is treated in the same manner as the detection of other less severe faults within the zone of protection and consequently the relay operated at the same speed in both situations.
Accordingly, it is an object of the present invention to provide improved discrimination between internal and external faults thereby increasing the reliability of relay operation and the security of the protection system.
Another object of the present invention is to minimize erroneous operation of the phase relay due to the presence of high frequency transients on the protected line.
A further object of the present invention is to prevent operation of the phase relay upon detection a close-in single phase to ground fault.
Yet another object of the present invention is to provide faster operating times upon detection of severe internal faults.
A further object of the present invention is to provide an improved polarizing signal using positive sequence current to derive a forward offset reach in order to reduce relay characteristic timer settings and obtain an improved steady state characteristic when the relay is used on a heavily loaded line and thereby increase relay operating speed.
These and other objects of the present invention will become apparent to those skilled in the art upon consideration of the following description of the invention.